2. Abstract
This report describes how lumber can be air-dried most
effectively under outdoor conditions and illustrates the
principles and procedures of air-drying lumber that were
developed through field investigations and observations of
industrial practices. Particular emphasis is placed on the
yarding of lumber in unit packages. Included are topics such
as why lumber is dried, advantages and limitations of the
drying process, properties of wood in relation to drying,
layout of the drying yard, piling methods, causes and reme-
dies of air-drying defects, and protection of air-dried lumber.
Keywords: drying lumber, air dry, wood structure, wood
shrinkage, drying rate, wood defects
October 1999
Forest Products Laboratory. 1999. Air drying of lumber. Gen. Tech. Rep.
FPL–GTR–117. Madison, WI: U.S. Department of Agriculture, Forest
Service, Forest Products Laboratory. 62 p.
A limited number of free copies of this publication are available to the
public from the Forest Products Laboratory, One Gifford Pinchot Drive,
Madison, WI 53705–2398. Laboratory publications are sent to hundreds
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Preface
This manual is a revision of the 1971 edition of Air Drying of
Lumber: A Guide to Industry Practice by Raymond C. Rietz
and Rufus H. Page. A major contributor to the 1971 edition
was Edward C. Peck, formerly a wood drying specialist at the
Forest Products Laboratory.
The major reason for this revison is the continued interest in
and requests for the manual, even almost 30 years after initial
publication. Because of this continued interest, we main-
tained the same basic framework in the revision. Our main
objective was to update with new or more current informa-
tion where necessary, keeping in mind that our audience will
range from small industry (perhaps new to lumber drying) to
larger industry (perhaps having substantial experience in air
drying). Involved in this revision were
William T. Simpson, Research Forest Technologist,
John L. Tschernitz, Chemical Engineer (retired), and
James J. Fuller, Research Forest Products Technologist,
of the Forest Products Laboratory.
Acknowledgments
Special recognition is given to Edward C. Peck, formerly a
Wood Drying Specialist at the USDA Forest Service, Forest
Products Laboratory, for his research and related contribu-
tions that formed the basis for most of this publication. We
also appreciate the cooperation of lumber-producing indus-
tries in providing photographs of their operations.
3. Air Drying of Lumber
USDA Forest Service
Forest Products Laboratory
Madison, Wisconsin
4. Contents
Page
Introduction .........................................................................1
1. Why Dry Lumber?..........................................................3
Drying Methods..............................................................3
Objectives.......................................................................4
Advantages and Limitations............................................6
2. Wood Properties and Moisture.......................................7
Structure..........................................................................7
Moisture Content ..........................................................10
Methods to Determine Moisture Content......................10
Moisture Movement......................................................12
Equilibrium Moisture Content ......................................14
Fiber Saturation Point...................................................14
Shrinkage......................................................................14
Weight ..........................................................................18
Color.............................................................................18
3. Air-Drying Process.......................................................21
Utilizing Air Movement................................................21
Factors That Influence Drying Rate..............................21
Drying Time and Final Moisture Content.....................23
Deterioration of Lumber...............................................25
4. Air-Drying Yard ...........................................................27
Yard Layout and Orientation ........................................27
Yard Transportation Methods.......................................29
Pile Foundations ...........................................................29
Shed Drying..................................................................31
Yard Operation and Maintenance.................................32
Page
5. Piling Methods For Air Drying.....................................33
Sorting Lumber.............................................................33
Sorting Equipment........................................................34
Stacking Lumber in Packages.......................................34
Stacking Random Length Lumber ................................38
Special Piling Methods.................................................38
Stickering Lumber ........................................................39
6. Air-Drying Defects—Causes and Remedies.................43
Chemical Reaction........................................................43
Fungal Infection............................................................43
Insect Infestation...........................................................44
Shrinkage......................................................................44
Warp Reduction............................................................48
Stacking, Package Piling, and Pile Protection ..............49
7. Protection of Air-Dried Lumber...................................51
Outdoors .......................................................................51
Indoors..........................................................................51
In Transit ......................................................................53
Appendix. Summarized Guide for Air-Drying Practices..55
Glossary.............................................................................58
5. Introduction
Trees contain a considerable amount of water. Most of this
water must be evaporated before the lumber obtained from
a tree can be converted into consumer products. The lumber
from which most wood products are manufactured must
be dried.
Of all the methods to remove large quantities of water from
wood, air drying has the least capital costs, especially in the
early stages of drying. However, air drying offers little
opportunity to control the drying process.
Drying wood by allowing natural forces to evaporate the
water is not a new idea. Over the years, people learned how
to utilize these forces effectively. However, this information
accumulated slowly, and many individuals are aware of only
part of the technology. This publication assembles informa-
tion on air drying learned through research and information
that has evolved from many years of experience.
This report gives plant managers, yard supervisors, lumber
handlers, and others in related areas an overview of the
principles of air drying. It will also assist them in analyzing
their air-drying practices. Drying wood by any means is
expensive, so operators are constantly on the alert to reduce
costs. Application of general air-drying principles may lead
to changes in yarding methods that will result in faster drying
of lumber and improved quality.
The results of efficient air drying benefit consumers of forest
products and are of key importance in utilization of the
Nation’s forest resource. Efficient air drying helps to con-
serve supplies of wood, thereby the timber resource, by
reducing loss of product from drying degrade. Air drying
also reduces the need to burn fuels for energy to dry lumber,
thus conserving these fuels and reducing atmospheric emis-
sions. In addition, air drying helps to ensure continued mar-
kets for wood products by contributing to customer satisfac-
tion. Both conservation of the timber resource and customer
satisfaction contribute to the wise use of timber, which has
long been an accepted tenet of the USDA Forest Service’s
conservation policy.
The emphasis of this publication is practicality. After touch-
ing on the variety of reasons for drying lumber and the wood
properties that relate to drying, the basics of the air-drying
process are presented. Then, descriptions are given of the
drying yard, methods of piling lumber for air drying, defects
that occur in air drying, and protection of the air-dried lum-
ber. Finally, an appendix summarizes the information in this
report and serves as a convenient means of checking specific
points in an individual operation. As in any industry, specific
terminology has evolved and shades of meaning show up
geographically, between hardwood and softwood production
or between a plant operator and a researcher. For this reason,
a glossary is included to delineate how words are used in this
publication. Wood species are generally listed in this report
by their common names. However, different common names
are often used for the same species; therefore, both common
and botanical names are given in Table 1.
6. 2
Table 1—Average moisture content of green wood, by species
Moisture contenta
(%) Moisture contenta
(%)
Species Botanical name
Heart-
wood
Sap-
wood
Mixed
heart-
wood
and
sap-
wood Species Botanical name
Heart-
wood
Sap-
wood
Mixed
heart-
wood
and
sap-
wood
Hardwoods Softwoods
Alder, red Alnus rubra — 97 — Baldcypress Taxodium distichum. 121 171 —
Ash, black Fraxinus pigra 95 — — Cedar, Alaska` Chamaecyparis nootkat-
ensis.
32 166 —
Ash, green F. pennsylvanica — 58 — Cedar, Atlantic white Chamaecyparis thy-
oides.
— — 35
Ash, white F. americana 46 44 — Cedar, eastern red Juniperus virginiana 33 — —
Aspen, bigtooth Populus grandidentata 95 113 — Cedar, incense Libocedrus decurrens. 40 213 —
Aspen, quaking P. tremuloides 95 113 — Cedar, Northern white Thuja occidentalis 32 240 93
Basswood, American Tilia americana 81 133 — Cedar, Port-Orford Chamaecyparis lawson-
iana.
50 98 —
Beech, American Fagus grandifolia 55 72 — Cedar, western red Thuja plicata 58 249 62
Birch, paper Betula papyrifera 89 72 — Douglas-fir, coast
type
b
Pseudotsuga menziesii 37 115 45
Birch, sweet B. lenta 75 70 — Douglas-fir, interme-
diate type
P. menziesii 34 154
Birch, yellow B. alleghaniensis 74 72 — Douglas-fir, Rocky
Mountain type
P. menziesii 30 112 43
Butternut Juglans cinerea — — 104 Fir, balsam Abies balsamea 88 173 117
Cherry, black Prunus serotina 58 — 65 Fir,, California red A. magnifica — — 108
Cottonwood, black Populus trichocarpa 162 146 — Fir, grand A. grandis 91 136 —
Cottonwood, eastern P. deltoides 160 145 — Fir, noble A. procera 34 115 —
Elm, American Ulmus americana 95 92 — Fir, Pacific silver A. amabilis 55 164 —
Elm, rock U. thomasii 44 57 — Fir, subalpine A. lasiocarpa — — 47
Hackberry Celtis occidentalis 61 65 — Fir, white A. concolor 98 160 —
Hickory, Carya spp 71 51 — Hemlock, eastern Tsuga canadensis 97 119 —
Magnolia, southern Magnolia grandiflora. 80 104 — Hemlock, western T. heterophylla 85 170 —
Maple, bigleaf Acer macrophyllum 77 138 — Larch, western Larix occidentalis. 54 119 —
Maple red A. rubrum — — 70 Pine, eastern white Pinus strobus 50 175 90
Maple, silver A. saccharinum 58 97 — Pine, jack P. banksiana — — 70
Maple, sugar A. saccharum 65 72 — Pine, lodgepole P. contorta 41 120 —
Oak, northern red Quercus rubra 80 69 — Pine, ponderosa P. ponderosa 40 148 —
Oak, northern white Q. alba 64 78 — Pine, red P. resinosa 32 134 —
Oak, southern red Q. falcata 83 75 — Pine, Southern,
loblolly
P. taeda 33 110 —
Oak, southern white
(chestnut)
Q. prinus 72 — — Pine, Southern,
longleaf
P. palustris 31 106 —
Pecan Carya illinoensis 71 62 — Pine, Southern,
shortleaf
P. echinata 32 122 —
Sweetgum Liquidambar styraciflua 79 137 — Pine, Southern, slash P. elliottii 30 100 —
Sycamore, American Platanus occidentalis 114 130 — Pine, sugar P. lambertiania 98 219 —
Tanoak Lithocarpus densiflorus — — 89 Pine, western white P. monticola 62 148 —
Tupelo, black Nyssa sylvatica 87 115 — Redwood, old growth Sequoia sempervirens. 86 210 —
Tupelo, water N. aquatica 150 116 — Spruce, black P. moriana 52 113 77
Walnut, black Juglans nigra 90 73 — Spruce, Engelmann Picea engelmannii 51 173 —
Willow, black Salix nigra — — 139 Spruce, red P. rubens — — 55
Yellow-poplar Liriodendron tulipifera. 83 106 — Spruce, Sitka P. sitchensis 41 142 43
Spruce, white P. glauca — — 55
a
Based on weight when ovendried.
b
Coast Douglas-fir is defined as Douglas-fir growing in the states of Oregon and Washington west of the summit of the Cascade Mountains. Interior West includes
the California and all counties in Oregon and Washington east of but adjacent to the Cascade summit. Interior North includes the remainder of Oregon and
Washington and the States of Idaho, Montana, and Wyoming. Interior South is made up of Utah, Colorado, Arizona, and New Mexico.
7. 3
Chapter 1
Why Dry Lumber?
Most water in the cut tree must be removed before useful
products can be made from the wood. The rough, green
lumber sawn from the log must be dried before it is proc-
essed into most end products. Drying the lumber at this stage
has a number of distinct and important advantages:
• Drying reduces weight, thereby reducing shipping and
handling costs.
• The shrinkage that accompanies drying takes place before
the wood is used as a product.
• As wood dries, most strength properties increase.
• The strength of joints made with nails and screws is greater
in dry wood than in green wood.
• Wood must be relatively dry before it can be glued or
treated with preservatives and fire-retardant chemicals.
• Drying reduces the likelihood of mold, stain, or decay.
• Drying increases thermal insulating properties and im-
proves finishing characteristics.
Drying Methods
Several methods can be used to dry lumber, ranging from air
and kiln drying to special seasoning processes. Basically, all
methods involve moving moisture from the inside of the
wood to the surface, where it is evaporated into the air. Heat
and air movement speed up the drying process. Although this
publication deals with air drying only, a brief description of
other major methods is included to clarify how they differ
from air drying.
Air Drying
To air dry, the lumber is arranged in layers, or courses, with
separating stickers, and built up into unit packages and piles
outdoors so that atmospheric air can circulate through the
piles and carry away moisture (Fig. 1). One modification of
air drying is shed drying, where the lumber to be dried is
placed in a shed having open sides (Fig. 2). The roofed
structure protects the lumber from rain and direct solar radia-
tion but allows outdoor air to circulate through the stickered
lumber to dry it.
Fan Shed Air Drying
To accelerate air drying, stickered unit packages of lumber
are placed in an unheated shed or building that has fans on
one side and is open on the other. The fans create air move-
ment through the spaces between the courses of wood
(Fig. 3).
Forced Air Drying and Predrying
In more complex drying processes, stickered packages of
lumber are placed in closed buildings that have fans to recir-
culate heated air through the lumber piles. Both forced air
dryers and predryers are commonly considered low tempera-
ture, forced-air circulation, ventilated dry kilns (Fig. 4).
Kiln Drying
To kiln dry, lumber is dried in a closed chamber by control-
ling the temperature, relative humidity, and air circulation
until the wood reaches a predetermined moisture content
(Fig. 5).
Special Drying Processes
To reduce drying time and degrade, several processes for
drying lumber have been investigated. Solar energy has
gained popularity with small drying operations. Vacuum
drying, especially when combined with radio frequency or
microwave delivery of energy to evaporate water, has also
gained attention in recent years. The main advantage of
vacuum drying is reduced drying time without increased
drying defects. Other special drying processes in limited use
are press drying between heated platens, solvent seasoning,
and boiling in oil.
Choice of Methods
Factors that determine the lumber drying process used at a
plant are generally related in one way or another to econom-
ics. A sawmill that produces a considerable volume of a
rather slow-drying wood (such as oak) often selects air dry-
ing followed by kiln drying. However, softwoods are often
kiln dried green from the saw.
8. 4
Figure 2—Shed for drying lumber protects the
lumber from the weather.
Objectives
The main purpose for air drying lumber is to evaporate as
much water as possible while minimizing capital expendi-
tures for dry-kiln capacity and without incurring a cost for
the energy required. In air drying, lumber is usually left on
stickers in the yard until it reaches a moisture content be-
tween 20% and 25%. The lumber may then be ready for
further processing, depending upon its end use. If it must be
dried to lower moisture content levels, such as for use in
furniture factories, the lumber will be kiln dried.
When lumber use does not require a low moisture content,
air drying is usually sufficient. Lumber used for outdoor
furniture and other outdoor exposures, or for building struc-
tures such as barns, pole sheds, and garages that are not
heated, can usually be air dried to a low enough moisture
content. Rough sawn hardwoods are often air dried at the
producing sawmills to reduce weight so that shipping costs
are reduced. In addition, air drying reduces subsequent kiln-
drying time.
The main benefit of the air-drying process, particularly for
hardwoods, is that it offers a way to add value while the
inventory of lumber is being held. To meet production of
shipping schedules during periods of the year when the saw-
mill cannot be operated to capacity, the yard inventory is
built up when sawing conditions are favorable and the lum-
ber is air dried while being held. Air drying is sometimes
used to reduce the moisture content in wood, such as railroad
ties, to a level suitable for preservative treatment.
Air drying further reduces the chance that mold and decay
may develop in transit, storage, or subsequent use. Blue stain
and wood-destroying fungi cannot grow in wood with a
moisture content of 20% or less. However, green lumber may
have to be treated with a fungicide to protect it from these
fungi in the early stages of the air-drying process. Drying is
also a protective measure against damage from most insects
that bore holes in wood.
Figure 1—An air-drying yard arranged for good circulation of air around the piles of packaged lumber.
9. 5
Figure 3—Fan shed air dryer draws air through the stickered packages of lumber.
Figure 4—Predryers are large structures that provide some means of controlling the drying
conditions. The forklift in the background provides size comparison.
10. 6
Advantages and Limitations
The greatest advantage of air drying lumber when compared
with drying by other processes is low capital costs. However,
as the value of the wood increases, kiln drying green wood
becomes more feasible. Species such as beech, birch, and
maple are often kiln dried green from the saw. The limita-
tions of air drying are associated with the uncontrollable
nature of the process. The drying rate is very slow during the
cold winter months in the northern sector of the country. At
other times, hot, dry winds may increase degrade and volume
losses as a result of severe surface checking and end splitting.
Production schedules depend on changing climatic condi-
tions of temperature, relative humidity, rainfall, sunshine,
and winds. Warm, humid, or sultry periods with little air
movement encourage the growth of blue stain and aggravate
chemical brown and gray stain.
Figure 5—Typical package-loaded dry kiln.
11. 7
Chapter 2
Wood Properties and Moisture
The structure of wood, the location and amount of moisture
contained in green wood, and the physical properties of wood
greatly influence its drying characteristics and reactions to
air-drying conditions.
Structure
Bark, Wood, and Pith
A cross section of a tree shows well-defined features from
the outside to the center (Fig. 6). The bark is divided into two
layers—the corky, outer dead portion and the inner living
portion. The light-colored zone of wood next to the bark is
called sapwood and the darker inner zone of wood is called
heartwood. In the center of the tree is a very small, soft core
known as pith.
Hardwoods and Softwoods
Typically, hardwoods are trees with broad leaves, and soft-
woods are trees with needle-like or scale-like leaves. These
terms do not apply to the hardness or density of the woods.
Some softwoods, such as southern yellow pine, are harder
than some hardwoods, such as basswood or cottonwood.
The structure of hardwoods is generally more complex than
that of softwoods. Figure 7 shows the vessels and other cells
in a hardwood cube highly magnified. Figure 8 shows a
similar cube of softwood. Many hardwoods contain relatively
large wood rays, and some softwoods contain resin ducts.
Both rays and resin ducts are related to increased susceptibil-
ity to surface checking during drying.
Cellular Structure
Wood is composed of hollow, tube-like cells called fibers,
usually closed at both ends. Thin spots or pit membranes are
located in the walls of cells, through which the sap flows in
the living tree or moisture moves during the drying of lum-
ber. Most cells lie nearly parallel to the long axis of the tree
trunk, but some lie perpendicular to the long axis of the tree
on radial lines from the pith to the bark. These cells are
called wood rays. Each year, several layers of new cells are
produced on the outside of the sapwood by the thin living
layer called the cambium.
Earlywood and Latewood
A cross section of a tree grown in a temperate climate shows
well-defined concentric layers of wood, which correspond
closely to yearly increments of growth. For that reason, they
are commonly called annual growth rings. Earlywood, some-
times called springwood, is formed during the early part of
each growing season. The cells of the earlywood are gener-
ally larger in cross section and thinner walled than those
formed later in the season. Consequently, earlywood is
softer, weaker, and generally lighter in color than latewood
or summerwood.
Figure 6—Cross section of an oak tree: A, Outer bark
or corky layer is composed of dry dead tissue, gives
general protection against external injuries. B, Inner
bark is moist and soft, carries prepared food from leaves
to all growing parts of the tree. C, Cambium layer is
inside of the inner bark and forms wood and bark cells.
D, Sapwood is the light-colored wood beneath the bark;
it carries sap from the roots to leaves. D, Heartwood
(inactive. is formed by a gradual change in the sapwood;
it gives the tree strength. F, Pith is the soft tissue about
which the first wood growth takes place in the newly
formed twigs. G, Wood rays are strips of cells that
extend radially within the tree and serve primarily to
store and transport food.
12. 8
Sapwood and Heartwood
In the living tree, the sapwood layer, which is near the bark,
contains many living cells that serve mainly in the transfer
and storage of food. Some of the cells that store food are
associated with stain. These cells remain alive after the tree is
cut, and they release enzymes that convert the sugars into a
brown chemical. However, most sapwood cells are dead and
serve only as channels for sap to move upward in the tree and
to help support the tree.
The central part of the trunk is called heartwood. All heart-
wood cells are dead, and their principal function is to supply
strength to the trunk and store extraneous materials. As a tree
increases in diameter by adding new layers of sapwood under
the bark, the zone of heartwood also enlarges at substantially
the same rate. The living cells of the sapwood die and be-
come infiltrated with gums, resins, coloring matter, and other
materials. The circumference of the heartwood may be ir-
regular and does not necessarily follow the annual growth
rings closely.
The relative amounts of sapwood and heartwood vary con-
siderably, both between species and in trees of the same
species. A tree of small diameter has more sapwood propor-
tionately than a similar tree of larger diameter. Within
a species, sapwood is thickest in the most vigorously
growing trees.
Heartwood, as a rule, is less permeable to liquids than is
sapwood. For this reason, heartwood dries more slowly than
does sapwood. In resinous woods, the heartwood usually
contains more resin than does the sapwood.
Juvenile Wood
The wood adjacent to the pith is called juvenile wood. This
wood shrinks more parallel to the grain than does the sur-
rounding wood and may contribute to warping longitudinally
(Fig. 9).
Reaction Wood
The growing tree develops wood with distinctive properties
in parts of leaning or crooked trunks and in branches. This
wood is called compression wood in softwoods and tension
wood in hardwoods.
Figure 7—A section of hardwood highly magnified:
rr, radial surface; tg, tangential surface; ar, annual
growth ring; ew, earlywood; lw, latewood; wr, wood
ray; wf, wood fiber; v, vessels or pores.
Figure 8—A section of a softwood highly magnified:
rr, radial surface; tg, tangential surface; ar, annual
growth ring; ew, earlywood; lw, latewood; wr, wood ray;
tr, trachied or fiber; vrd, vertical resin ducts. The large
hole near the center of the top section and the passage
along the right edge (vrd) are vertical resin ducts.
13. 9
Compression wood (Fig. 10) occurs on the underside of the
trunks of leaning softwood trees and on the underside of
limbs. Annual growth rings in compression wood are usually
wider than normal rings; latewood rings are unusually wide
but do not appear as dense as normal latewood. A lack of
color contrast between earlywood and latewood gives a
lifeless appearance to compression wood. It is usually yel-
lowish or brownish in color and may also have a reddish
tinge. Streaks of compression wood are frequently inter-
spersed with normal wood. Compression wood shrinks more
longitudinally than does normal wood and may cause warp-
ing (Fig. 11) or develop cross breaks (Fig. 12) during drying.
Tension wood occurs in hardwoods, on the upper side of
leaning tree trunks and on the upper side of limbs. It can
cause a stressed condition in the log and later contributes to
splitting in sawn products. Tension wood also shrinks ab-
normally, thus possibly contributing to warp during drying.
When lumber is machined, zones of tension wood are
indicated by torn grain (Fig. 13).
Structural Irregularities
The length of the wood cells is usually parallel to the length
of the tree trunk. Sometimes the length of the wood cells
forms an angle with the length of the tree trunk. If this orien-
tation of the fibers continues around the circumference and
upward in the trunk, spiral grain results. Spiral grain shows
up on the surface of boards as grain that is at an angle to the
edge of the board. Another cause for disoriented grain is a
log that is sawn parallel to the pith rather than to the bark,
and these boards will have a diagonal grain. Both spiral and
diagonal grain are termed cross grain or slope of grain in
boards or other sawn products. The principal drying defect
resulting from cross grain is warp. Cross grain also causes
mechanical weakness.
Figure 9—When the board was ripped in two, each piece crooked because of longitudinal shrinkage of juvenile
wood in the center of the board.
Figure 10—Eccentric growth around the pith in a cross
section containing compression wood. The dark area
in the lower third of the cross section is compression
wood.
Figure 11—Crook resulted when the band of com-
pression wood (darker strip running length of piece)
shrank more longitudinally than did the lighter colored
normal wood.
Figure 12—Board with dark band of compression wood
with cross breaks caused by longitudinal shrinkage.
14. 10
Knots
A knot is revealed when lumber is cut from the portion of a
tree containing an embedded branch. Normally, a knot starts
at the pith and grows outward through the bark. Knots are
generally objectionable because the distortion and the
discontinuity of the grain around knots weakens the wood.
Furthermore, knots cause irregular shrinkage and warping.
When lumber dries, knots and the wood adjacent to them
tend to check. Knots can also become loose because of their
change in size. In addition, loose knots are formed when a
tree grows around a dead branch. When the lumber is sawn,
the dead knot may fall out because it is not connected to the
rest of the tree.
Moisture Content
The liquid in freshly cut lumber is often called sap. Sap is
composed primarily of water, with varying amounts of other
dissolved materials. Moisture in wood exists in two forms: as
free water in the cell cavities and as bound moisture held
within the cell walls. Sapwood usually contains more
moisture than does heartwood, particularly in softwoods.
Table 1 lists moisture content values of green wood for
several species.
The moisture content of wood can vary at different heights in
the tree; species and growing conditions are involved. Butt
logs of sugar pine, western larch, redwood, and western red
cedar sometimes sink in water because their density at these
high moisture content levels exceed that of water, although
the upper logs from the same trees float. In addition to hav-
ing a higher moisture content, these “sinker” logs can have a
higher specific gravity or more wood substance per unit
volume.
Methods to Determine
Moisture Content
The performance of wood is influenced by the amount of
moisture it contains; for many uses, wood serves best at
specific levels of moisture content. Therefore, knowing the
moisture content is an essential part of determining the readi-
ness of wood for use. The amount of moisture in wood is
expressed as a percentage of the weight of the dry wood
substance. Moisture content of wood can be determined by
the oven-drying method, a distillation method, or the use of
electric moisture meters (Figs.14,15).
Oven-Drying Method
The oven-drying method is the standard way of determining
the moisture content of wood. A cross section is cut from a
board, then completely dried in a heated oven. The method
consists of the five following steps:
1.Cut a cross section from a board about 25 mm (1 in.) thick
along the grain.
2.Immediately after sawing, remove all loose splinters and
weigh the section.
3.Put the section in an oven maintained at 105o
C (220o
F) and
dry until constant weight is attained, usually about 24 h.
4.Weigh the dried section to obtain the ovendry weight.
5.Subtract the ovendry weight from the initial weight and
divide the difference by the ovendry weight, multiplying
the result by 100 to obtain the percentage of moisture in
the section:
moisture content (%) = 100 × (initial weight − ovendry
weight)/ ovendry weight
A short-cut formula convenient to use is
moisture content (%) = 100 × (initial weight/ovendry
weight −1)
Several types of balances are used in weighing specimens to
determine moisture content. One inexpensive type is the
triple beam balance (Fig 16). A direct reading automatic
balance is very convenient (Fig. 16) when several specimens
are weighed in and out of the drying oven. Self-calculating
Figure 13—Area of tension wood is indicated by grain
torn during machining.
15. 11
balances are also available (Fig. 17). The moisture content
calculation is carried out on the balance by following the
prescribed sequence of operations supplied by the manufac-
turer. The ovens used for drying the moisture sections should
be large enough to accommodate several specimens with
space between them. Ovens should also be well ventilated to
allow the evaporated moisture to escape. The temperature of
the oven must be controlled with a reliable thermostat. Ex-
cessive temperatures will char the specimens, introducing
errors in the moisture analysis.
Electrical Methods
Electric moisture meters permit the determination of mois-
ture content without cutting or seriously marring the board.
Such meters are rapid and reasonably accurate through the
range from 7% to 25% moisture content. Several tests can be
made with the electric moisture meter on a board or a lot of
lumber to arrive at a better overall average, canceling out the
inaccuracies that might exist in individual measurements.
With some types of electrodes, a minor consideration is that
small needlepoint holes are made in the lumber being tested.
Electric moisture meters are extensively used, particularly the
portable or hand meters. Two types, each based on a differ-
ent fundamental relationship, have been developed: (1) the
resistance, or pin type, which uses the relationship between
moisture content and electrical resistance (Fig. 14); and
(2) the capacitance type, which uses the relationship between
moisture content and the dielectric properties of wood and its
included water (Fig. 15).
Portable, battery-operated, resistance-type moisture meters
are wide range ohmmeters. Most models have a direct read-
ing meter, calibrated in percentage for one species; the
manufacturer provides corrections for other species, either
Figure 14—Conductance-type electrical moisture meters.
16. 12
built into the meter or in the form of tables. The manufac-
turer also provides a temperature correction chart or table for
correcting the meter reading when tests are made on wood
warmer than 32o
C (90°F) or cooler than 21o
C (70°F).
Resistance-type meters are generally supplied with two pin-
type electrodes that are driven into the wood being tested.
Lengthy, two-pin electrodes that are insulated except for the
tip are also available for use on lumber thicker than 41 mm
(1-5/8 in.). Valid estimates of the average moisture content
of the drying board are obtained by driving the pins deep
enough in the lumber so that the tip reaches a fifth to a fourth
of the thickness of the board. If the surface of the lumber has
been wetted by rain, the meter indications are likely to be
much greater than the actual average moisture content unless
insulated shank pins are used.
Capacitance-type hand meters use surface contact-type elec-
trodes. The electric field radiating from the electrode pene-
trates about 19 mm (3/4-in.) into the wood so that lumber
thicknesses to about 38 mm (1-1/2 in.) can be tested. How-
ever, the moisture content of the surface layers of the lumber
has a predominant effect on the meter readings, simply be-
cause the electric field is stronger near the surface in contact
with the electrode.
Temperature correction charts or tables are not provided by
the manufacturers of capacitance-type meters. However, they
do provide species correction tables for converting the meter
scale reading of the instrument to moisture content for
individual species.
Moisture Movement
During air drying, water at or near the lumber surface evapo-
rates first. Then, water deeper in the wood moves from zones
of high moisture content to zones of lower moisture content
in an effort to reach a moisture equilibrium throughout the
board. This means that moisture from the interior of a wet
board moves to the drier surface zones. Outdoor air must not
be restricted from circulating over the green lumber to dry
the surfaces and draw moisture from the interior of the board.
Moisture moves through several kinds of passageways in the
wood. The principal ones are the cavities in the cells, the pit
chambers, and pit membrane openings in the cell walls.
Movement of moisture in these passageways occurs not only
lengthwise in the cells but also sideways from cell to cell
through pit membranes toward the drier surfaces of the wood.
When wood dries, several moisture-driving forces may be
operating to reduce its moisture content. These forces, which
may be acting at the same time, include the following:
• Capillary action that causes the free water to move through
the cell cavities, pit chambers, and pit membrane openings
(the same action that causes a liquid to wet a wick)
• Differences in relative humidity that cause moisture in the
vapor state to flow through cell cavities, pit chambers, pit
membrane openings, and intercellular spaces to regions of
lower humidity
• Moisture content differences that cause movement of
moisture to regions of lower moisture content through the
passageways within the cell walls
Figure 15—Capacitance-type electrical moisture meter.
17. 13
When green wood starts to dry, evaporation of water from
openings in the surface cells creates a capillary pull on the
free water in the cell cavity and in adjacent cells. Free water
moves from one tubular cell to another toward the wood
surface by capillary action. When the free water has evapo-
rated in the cells, the moisture remaining is in the form of
vapor in the cell cavities or bound water in the cell walls.
The movement of water vapor through void spaces in wood
depends on how much water vapor is contained in the air in
the voids or in the air surrounding the wood. If the air sur-
rounding the wood has a low relative humidity, water vapor
will move from the wet wood to the air. Thus, rapid drying
depends on the surface moisture content of the wood being
dried and whether a difference in moisture content can be
developed between the surface and the interior of a board.
The higher the temperature to which the drying wood is
subjected, the faster the combined vapor and moisture
movement. This is why the air-drying rate is faster in summer
than in winter.
Figure 16—Three types of balances that can be used to weigh moisture sections and kiln samples:
(a) triple beam balance for moisture sections, (b) electronic top loading balance for moisture sections,
(c) electronic top loading balance for kiln samples, (d) non-electronic balance for kiln samples.
Figure 17—Self-calculating moisture content balance.
The triple beam balance is provided with a special scale
on the specimen pan that is used to calculate the
moisture content after the section is ovendried.
18. 14
Equilibrium Moisture Content
Wood is a hygroscopic material. It gives off or takes on
moisture until it is in equilibrium with the relative humidity
of the surrounding air. When the moisture content of the
wood has reached equilibrium, it is said to have reached its
equilibrium moisture content (EMC). The EMC of wood can
be predicted by knowing the temperature and relative humid-
ity conditions of the air in contact with the surface of the
drying lumber; this relationship is shown in Table 2. Table 3
shows the average monthly EMC values for various cities in
the United States.
The air temperature in a lumber drying yard is determined
with a dry-bulb thermometer. To determine the relative
humidity of the air, a wet-bulb thermometer can also be used.
If the two thermometers are mounted on a single base, the
instrument is called a hygrometer (Fig. 18). The wet-bulb
thermometer has the bulb covered with a clean, soft cloth
wick that dips into a reservoir of pure, clean water. Water in
the reservoir wets the wick by capillary action. As a result of
the cooling caused by evaporation from the wet surface of the
wick, the wet-bulb thermometer will give a lower reading
than the dry-bulb thermometer. This difference in readings,
called the wet-bulb depression, is a measure of the relative
humidity of the air. To obtain a true reading with the wet-
bulb thermometer, the hygrometer must be placed in a strong
current of air. Table 4 gives relative humidity and EMC
values for various dry-bulb temperatures and wet-bulb de-
pressions. In addition to wet- and dry-bulb thermometers,
direct reading hygrometers are also available, although they
may not be as accurate as the measurement using wet- and
dry-bulb thermometers.
Fiber Saturation Point
As previously stated, the moisture in the cell cavity is called
free water, and that held in the cell walls is called bound
water. Conceptually, the moisture content when all the free-
water in the cell cavity is removed but the cell wall remains
saturated is called the fiber saturation point (fsp). For most
practical purposes, this condition exists at a cell moisture
content of about 30%. Note that the definition of fsp applies
to cells or localized regions but not to the moisture content of
an entire board. During drying, the center of a board may be
at a relatively high moisture content (for example, 40%), and
the surface fibers may be much dryer (for example, 12%).
Overall, this board may be at an average moisture content of
30%, but it is not correct to say that the entire board is at fsp.
Comparatively large changes in the physical and mechanical
properties of wood occur with changes of moisture content
around the fsp. The moisture removed below this point
comes from the cell wall; therefore, the fsp is the moisture
content level at which shrinkage begins. More energy is
required to evaporate the bound water, because the attraction
between the wood and water must be overcome.
Shrinkage
When the cells in the surface layers of a board dry below the
fsp, about 30%, the cell walls shrink. Shrinkage of cells in
the surface region of the board can be sufficient to squeeze
the core of the board and cause a slight overall shrinkage of
the board. For most practical purposes, the shrinkage of
wood is considered as being directly proportional to the
amount of moisture lost below 30%. Shrinkage varies with
the species and the orientation of the fibers in the piece.
Normally, shrinkage is expressed as a percentage of the
green dimension. The reduction in size parallel to the growth
ring, or circumferentially, is called tangential shrinkage. The
reduction in size parallel to the wood rays, or radially, is
called radial shrinkage. Tangential shrinkage is about twice
as great as radial shrinkage in most species. This explains the
characteristic shrinkage and distortion of several wood
shapes shown in Figure 19.
Table 2—Moisture content of wood in equilibrium with stated dry-bulb temperature and relative humidity
Moisture content (%) for the following relative humidity valuesDry-bulb
temperature
(°C (°F)) 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95
–1.1 (30) 1.4 2.6 3.7 4.6 5.5 6.3 7.1 7.9 8.7 9.5 10.4 11.3 12.4 13.5 14.9 16.5 18.5 21.0 24.3
4.4 (40) 1.4 2.6 3.7 4.6 5.5 6.3 7.1 7.9 8.7 9.5 10.4 11.3 12.3 13.5 14.9 16.5 18.5 21.0 24.3
10.0 (50) 1.4 2.6 3.6 4.6 5.5 6.3 7.1 7.9 8.7 9.5 10.3 11.2 12.3 13.4 14.8 16.4 18.4 20.9 24.3
15.6 (60) 1.3 2.5 3.6 4.6 5.4 6.2 7.0 7.8 8.6 9.4 10.2 11.1 12.1 13.3 14.6 16.2 18.2 20.7 24.1
21.1 (70) 1.3 2.5 3.5 4.5 5.4 6.2 6.9 7.7 8.5 9.2 10.1 11.0 12.0 13.1 14.4 16.0 17.9 20.5 23.9
26.7 (80) 1.3 2.4 3.5 4.4 5.3 6.1 6.8 7.6 8.3 9.1 9.9 10.8 11.7 12.9 14.2 15.7 17.7 20.2 23.6
32.2 (90) 1.2 2.3 3.4 4.3 5.1 5.9 6.7 7.4 8.1 8.9 9.7 10.5 11.5 12.6 13.9 15.4 17.3 19.8 23.3
37.8 (100) 1.2 2.3 3.3 4.2 5.0 5.8 6.5 7.2 7.9 8.7 9.5 10.3 11.2 12.3 13.6 15.1 17.0 19.5 22.9
43.3 (110) 1.1 2.2 3.2 4.0 4.9 5.6 6.3 7.0 7.7 8.4 9.2 10.0 11.0 12.0 13.2 14.7 16.6 19.1 22.4
48.9 (120) 1.1 2.1 3.0 3.9 4.7 5.4 6.1 6.8 7.5 8.2 8.9 9.7 10.6 11.7 12.9 14.4 16.2 18.6 22.0
19. 15
Table 3—Average monthly EMC values for various U.S. citiesa
Equilibrium moisture content (%)
State City Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.
AK Juneau 16.5 16.0 15.1 13.9 13.6 13.9 15.1 16.5 18.1 18.0 17.7 18.1
AL Mobile 13.8 13.1 13.1 13.3 13.4 13.3 14.2 14.4 13.9 13.0 13.7 14.0
AZ Flagstaff 11.8 11.4 10.8 9.3 8.8 7.5 9.7 11.1 10.3 10.1 10.8 11.8
AZ Phoenix 9.4 8.4 7.9 6.1 5.1 4.6 6.2 6.9 6.9 7.0 8.2 9.5
AR Little Rock 13.8 13.2 12.8 13.1 13.7 13.1 13.3 13.5 13.9 13.1 13.5 13.9
CA Fresno 16.4 14.1 12.6 10.6 9.1 8.2 7.8 8.4 9.2 10.3 13.4 16.6
CA Los Angeles 12.2 13.0 13.8 13.8 14.4 14.8 15.0 15.1 14.5 13.8 12.4 12.1
CO Denver 10.7 10.5 10.2 9.6 10.2 9.6 9.4 9.6 9.5 9.5 11.0 11.0
DC Washington 11.8 11.5 11.3 11.1 11.6 11.7 11.7 12.3 12.6 12.5 12.2 12.2
FL Miami 13.5 13.1 12.8 12.3 12.7 14.0 13.7 14.1 14.5 13.5 13.9 13.4
GA Atlanta 13.3 12.3 12.0 11.8 12.5 13.0 13.8 14.2 13.9 13.0 12.9 13.2
HI Honolulu 13.3 12.8 11.9 11.3 10.8 10.6 10.6 10.7 10.8 11.3 12.1 12.9
ID Boise 15.2 13.5 11.1 10.0 9.7 9.0 7.3 7.3 8.4 10.0 13.3 15.2
IL Chicago 14.2 13.7 13.4 12.5 12.2 12.4 12.8 13.3 13.3 12.9 14.0 14.9
IN Indianapolis 15.1 14.6 13.8 12.8 13.0 12.8 13.9 14.5 14.2 13.7 14.8 15.7
IA Des Moines 14.0 13.9 13.3 12.6 12.4 12.6 13.1 13.4 13.7 12.7 13.9 14.9
KS Wichita 13.8 13.4 12.4 12.4 13.2 12.5 11.5 11.8 12.6 12.4 13.2 13.9
KY Louisville 13.7 13.3 12.6 12.0 12.8 13.0 13.3 13.7 14.1 13.3 13.5 13.9
LA New Orleans 14.9 14.3 14.0 14.2 14.1 14.6 15.2 15.3 14.8 14.0 14.2 15.0
ME Portland 13.1 12.7 12.7 12.1 12.6 13.0 13.0 13.4 13.9 13.8 14.0 13.5
MA Boston 11.8 11.6 11.9 11.7 12.2 12.1 11.9 12.5 13.1 12.8 12.6 12.2
MI Detroit 14.7 14.1 13.5 12.6 12.3 12.3 12.6 13.3 13.7 13.5 14.4 15.1
MN Minneapolis–
St.Paul
13.7 13.6 13.3 12.0 11.9 12.3 12.5 13.2 13.8 13.3 14.3 14.6
MS Jackson 15.1 14.4 13.7 13.8 14.1 13.9 14.6 14.6 14.1 14.1 14.3 14.9
MO St. Louis 14.5 14.1 13.2 12.4 12.8 12.6 12.9 13.3 13.7 13.1 14.0 14.9
MT Missoula 16.7 15.1 12.8 11.4 11.6 11.7 10.1 9.8 11.3 12.9 16.2 17.6
NE Omaha 14.0 13.8 13.0 12.1 12.6 12.9 13.3 13.8 14.0 13.0 13.9 14.8
NV Las Vegas 8.5 7.7 7.0 5.5 5.0 4.0 4.5 5.2 5.3 5.9 7.2 8.4
NV Reno 12.3 10.7 9.7 8.8 8.8 8.2 7.7 7.9 8.4 9.4 10.9 12.3
NM Albuquerque 10.4 9.3 8.0 6.9 6.8 6.4 8.0 8.9 8.7 8.6 9.6 10.7
NY New York 12.2 11.9 11.5 11.0 11.5 11.8 11.8 12.4 12.6 12.3 12.5 12.3
NC Raleigh 12.8 12.1 12.2 11.7 13.1 13.4 13.8 14.5 14.5 13.7 12.9 12.8
ND Fargo 14.2 14.6 15.2 12.9 11.9 12.9 13.2 13.2 13.7 13.5 15.2 15.2
OH Cleveland 14.6 14.2 13.7 12.6 12.7 12.7 12.8 13.7 13.8 13.3 13.8 14.6
OK Oklahoma City 13.2 12.9 12.2 12.1 13.4 13.1 11.7 11.8 12.9 12.3 12.8 13.2
OR Pendleton 15.8 14.0 11.6 10.6 9.9 9.1 7.4 7.7 8.8 11.0 14.6 16.5
OR Portland 16.5 15.3 14.2 13.5 13.1 12.4 11.7 11.9 12.6 15.0 16.8 17.4
PA Philadelphia 12.6 11.9 11.7 11.2 11.8 11.9 12.1 12.4 13.0 13.0 12.7 12.7
SC Charleston 13.3 12.6 12.5 12.4 12.8 13.5 14.1 14.6 14.5 13.7 13.2 13.2
SD Sioux Falls 14.2 14.6 14.2 12.9 12.6 12.8 12.6 13.3 13.6 13.0 14.6 15.3
TN Memphis 13.8 13.1 12.4 12.2 12.7 12.8 13.0 13.1 13.2 12.5 12.9 13.6
TX Dallas–Ft.Worth 13.6 13.1 12.9 13.2 13.9 13.0 11.6 11.7 12.9 12.8 13.1 13.5
TX El Paso 9.6 8.2 7.0 5.8 6.1 6.3 8.3 9.1 9.3 8.8 9.0 9.8
UT Salt Lake City 14.6 13.2 11.1 10.0 9.4 8.2 7.1 7.4 8.5 10.3 12.8 14.9
VA Richmond 13.2 12.5 12.0 11.3 12.1 12.4 13.0 13.7 13.8 13.5 12.8 13.0
WA Seattle–Tacoma 15.6 14.6 15.4 13.7 13.0 12.7 12.2 12.5 13.5 15.3 16.3 16.5
WI Madison 14.5 14.3 14.1 12.8 12.5 12.8 13.4 14.4 14.9 14.1 15.2 15.7
WV Charleston 13.7 13.0 12.1 11.4 12.5 13.3 14.1 14.3 14.0 13.6 13.0 13.5
WY Cheyenne 10.2 10.4 10.7 10.4 10.8 10.5 9.9 9.9 9.7 9.7 10.6 10.6
a
EMC values were determined from the average of 30 or more years of relative humidity and temperature data available
from the National Climatic Data Center of the National Oceanic and Atmospheric Administration.
20. 16
A flatsawn (or plainsawn) board (Fig. 20) shrinks tangen-
tially in width and radially in thickness. A quartersawn (or
vertical grain) board (Fig. 20) shrinks radially in width,
tangentially in thickness. Table 5 gives tangential and radial
shrinkage values for the wood of many species. The longitu-
dinal shrinkage of wood is generally slight, 0.1% to 0.2% of
the green dimension. If reaction wood and juvenile wood are
in the board, the longitudinal shrinkage may be appreciably
increased.
The values in Table 5 can be converted into dimensional
changes by using the following formula:
I
RT
FI
30
or
30
)(
M
SS
DMM
S
+
−
−
=
where S is shrinkage or swelling (mm, in., or any linear unit);
MI is initial moisture content (%); MF is final moisture con-
tent (%); D is dimension at initial moisture content (linear
units); 30 is fiber saturation point (%); ST and SR are tangen-
tial and radial shrinkage divided by 100. Neither the initial
nor the final moisture content can be greater than 30%. Ex-
amples using this formula to determine dimensional changes
are given in the following:
Example 1: Determine the shrinkage in width of a 152-mm
(6-in.), flatsawn, sugar maple board in drying from the green
condition to a moisture content of 15%. The average green
moisture content of sugar maple, sapwood and heartwood
combined, is about 69% (Table 1). This board is flat grained;
therefore, use a total tangential shrinkage value of 9.9%
(Table 5). Substituting in the formula:
mm5.7
303
2280
3030
099.0
30
152)1530(
==
+
−
−
=S
.in30.0
303
90
3030
099.0
30
6)1530(
==
+
−
−
=S
Example 2: Determine the shrinkage in width of a vertical
grained, old-growth redwood board, 8.5 in. (216 mm) wide,
in drying from a moisture content of 24% to 5%. Table 5
gives the radial shrinkage of redwood as 2.6%.
mm6.3
8.1147
4104
2430
026.0
30
216)524(
==
+
−
−
=S
.in14.0
8.1147
5.161
2430
026.0
30
5.8)524(
==
+
−
−
=S
Figure 18—(a) Stationary dry- and wet-bulb thermo-
meters indicating 24
o
C (75
o
F) dry-bulb and 17
o
C (63
o
F)
wet-bulb, for a relative humidity of 66% and EMC of
12.0% (Table 3). (b) A sling psychrometer to enhance
air velocity over the wet-bulb wick for greater accuracy.
22. 18
Example 3: Determine the swelling in width of a flatsawn,
sugar maple flooring strip machined to 57 mm (2.25 in.), in
changing from 5% to 13% moisture content.
mm6.1
278
456
530
099.0
30
57)135(
−=
−
=
+
−
−
=S
in.065.0
278
0.18
530
099.0
30
25.2)135(
−=
−
=
+
−
−
=S
The negative shrinkage obtained denotes swelling. Using the
swelling of 1.644 mm per 57.15 mm (0.0647 in. per 2.25 in).
of width, a 12.2-m- (40-ft-) wide floor will swell
mm35110002.12
15.57
644.1
=××
in.8.131240
25.2
0647.0
=××
Weight
The weight of lumber decreases as its moisture content is
reduced. One benefit of air drying is a reduction in shipping
weight. All wood substance, containing no air spaces and
regardless of the species from which it comes, has an oven-
dry specific gravity of approximately 1.54. This means that
0.0283 m3
(1 ft3
) of ovendry wood substance weighs
1.54 times the weight of 0.0283 m3
(1 ft3
) of water, or
28.4 ×1.54 = 43.7 kg (62.5 × 1.54 = 96.3 lb). Usually, the
specific gravity of wood is based on the volume of the wood
when green and its weight when ovendry. Thus, if the spe-
cific gravity of a specimen of green wood is listed as being
0.5, the ovendry weight of the wood substance in 0.0283 m3
(1 ft3
) of green wood is half the weight of a cubic foot of
water, or 14 kg (31.3 lb).
The differences in specific gravity between species are due to
differences in the size of the cells and the thickness of the
cell walls. The average specific gravity, based on ovendry
weight and green volume of commercial woods, varies from
0.31 for western red cedar and black cottonwood to 0.64 for
the hickories, and can be as high as 1.1 for some tropical
species. The moisture in the wood adds to the weight of a
given volume. Most species of wood, even when green, float
in water because of air entrapped in the cells.
Table 6 gives the specific gravity and lumber weight of
commercial species per 2.36 m3
(1,000 board feet) at a mois-
ture content of 25%.
Color
The sapwood of most species is light colored and ranges in
color from white to yellowish white. The heartwood of most
species is distinctly darker than the sapwood. Considerable
variation exists in the heartwood color of various species.
The colors are generally a combination of yellow, brown, and
red. Sometimes a greenish or purplish tint is present. The
color of wood exposed to air and light becomes duller and
darker. In air-drying yards, wood also becomes discolored by
ultraviolet degradation, wetting, and airborne dust. These
naturally caused surface discolorations do not penetrate far
into the wood, except into surface checks, and can be re-
moved by planing the wood.
Figure 19—Characteristic shrinkage and distortion of
flats, squares, and rounds as affected by the direction
of annual growth rings.
Figure 20—Shrinkage direction in (a) flatsawn
(plainsawn) and (b) quartersawn (vertical grain) boards.
23. 19
Table 5—Shrinkage values of domestic woods
a
Shrinkage
a
(%) from green
to ovendry moisture content
Shrinkage
a
(%) from green
to ovendry moisture content
Species Radial Tangential Species Radial Tangential
Hardwoods Softwoods
Alder, red 4.4 7.3 Baldcypress 3.8 6.2
Ash Cedar
Black 5.0 7.8 Alaska 2.8 6.0
Green 4.6 7.1 Atlantic white 2.9 5.4
White 4.9 7.8 Eastern redcedar 3.1 4.7
Aspen Incense 3.3 5.2
Bigtooth 3.3 7.9 Northern white 2.2 4.9
Quaking 3.5 6.7 Port-Orford 4.6 6.9
Basswood, American 6.6 9.3 Western redcedar 2.4 5.0
Beech, American 5.5 11.9 Douglas-fir
Birch Coast
b
4.8 7.6
Paper 6.3 8.6 Interior north
b
3.8 6.9
Sweet 6.5 9.0 Interior west
b
4.8 7.5
Yellow 7.3 9.5 Fir
Butternut 3.4 6.4 Balsam 2.9 6.9
Cherry, black 3.7 7.1 California red 4.5 7.9
Cottonwood Grand 3.4 7.5
Black 3.6 8.6 Noble 4.3 8.3
Eastern 3.9 9.2 Pacific silver 4.4 9.2
Elm Subalpine 2.6 7.4
American 4.2 9.5 White 3.3 7.0
Rock 4.8 8.1 Hemlock
Hackberry 4.8 8.9 Eastern 3.0 6.8
Hickory 7.4 11.4 Western 4.2 7.8
Magnolia, southern 5.4 6.6 Larch, western 4.5 9.1
Maple Pine
Bigleaf 3.7 7.1 Eastern pine 2.1 6.1
Red 4.0 8.2 Jack 3.7 6.6
Silver 3.0 7.2 Lodgepole 4.3 6.7
Sugar 4.8 9.9 Ponderosa 3.9 6.2
Oak Pine
Northern red 4.0 8.6 Red 3.8 7.2
Northern white 5.6 10.5 Southern
Southern red 4.7 11.3 Loblolly 4.8 7.4
Southern white (chest-
nut)
5.3 10.8 Longleaf 5.1 7.5
Pecan 4.9 8.9 Shortleaf 4.6 7.7
Sweetgum 5.3 10.2 Slash 5.4 7.6
Sycamore, American 5.0 8.4 Sugar 2.9 5.6
Tanoak 4.9 11.7 Western white 4.1 7.4
Tupelo Redwood
Black 5.1 8.7 Old growth 2.6 4.4
Water 4.2 7.6 Young growth 2.2 4.9
Walnut, black 5.5 7.8 Spruce
Willow, black 3.3 8.7 Engelmann 3.8 7.1
Yellow-poplar 4.6 8.2 Red 3.8 7.8
Sitka 4.3 7.5
White 4.7 8.2
a
Expressed as a percentage of the green dimension.
24. 20
Table 6—Average specific gravity and approximate weight of 2.36 m
3
(1,000 board feet) of various
species of lumber at 25% moisture content
Species
Average
specific
gravity
a
Average weight
of 2.36 m
3
(1,000 board
feet) (kg (lb)) Species
Average
specific
gravity
a
Average weight
of 2.36 m
3
(1,000 board
feet) (kg (lb))
Hardwoods Softwoods
Alder, red 0.37 1,114 (2,456) Baldcypress 0.42 1,260 (2,778)
Ash Cedar
Black 0.45 1,361 (3,001) Alaska 0.42 1,257 (2,772)
Green 0.53 1,595 (3,518) Atlantic white 0.31 927 (2,045)
White 0.55 1,658 (3,656) Eastern redcedar 0.44 1,315 (2,899)
Aspen Incense 0.35 1,045 (2,305)
Bigtooth 0.38 1,083 (2,387) Northern white 0.29 865 (1,908)
Quaking 0.35 1,053 (2,321) Port-Orford 0.40 1,169 (2,578)
Basswood, American 0.32 972 (2,143) Western redcedar 0.31 925 (2,039)
Beech, American 0.56 1,700 (3,748) Douglas-fir
Birch Coast
b
0.45 1,355 (2,987)
Paper 0.48 1,458 (3,216) Interior north
b
0.45 1,351 (2,978)
Sweet 0.60 1,816 (4,005) Interior west
b
0.46 1,383 (3,050)
Yellow 0.55 1,668 (3,678) Fir
Butternut 0.36 1,081 (2,383) Balsam 0.34 991 (2,186)
Cherry, black 0.47 1,413 (3,115) California red 0.36 1,081 (2,384)
Cottonwood Grand 0.35 1,051 (2,317)
Black 0.31 934 (2,059) Noble 0.37 1,114 (2,456)
Eastern 0.37 1,111 (2,450) Pacific silver 0.40 1,206 (2,659)
Elm Subalpine 0.31 929 (2,048)
American 0.46 1,390 (3,065) White 0.37 1,109 (2,445)
Rock 0.57 1,723 (3,799) Hemlock
Hackberry 0.49 1,478 (3,260) Eastern 0.38 1,138 (2,510)
Hickory 0.64 1,944 (4,287) Western 0.42 1,265 (2,788)
Maple Larch, western 0.48 1,448 (3,194)
Bigleaf 0.44 1,322 (2,916) Pine
Red 0.49 1,475 (3,253) Eastern white 0.34 1,016 (2,241)
Silver 0.44 1,324 (2,919) Lodgepole 0.38 1,142 (2,518)
Sugar 0.56 1,692 (3,732) Ponderosa 0.38 1,138 (2,510)
Oak Red 0.41 1,232 (2,716)
Northern red 0.56 1,689 (3,725) Southern
Northern white 0.60 1,818 (4,008) Loblolly 0.47 1,414 (3,118)
Southern red 0.52 1,575 (3,472) Longleaf 0.54 1,625 (3,583)
Southern white
(chestnut)
0.57 1,818 (4,009) Shortleaf 0.46 1,414 (3,118)
Pecan 0.60 1,809 (3,990) Sugar 0.35 1,015 (2,239)
Sweetgum 0.46 1,393 (3,071) Western white 0.36 1,052 (2,320)
Sycamore, American 0.46 1,388 (3,061) Redwood
Tanoak 0.56 1,760 (3,881) Old growth 0.38 1,133 (2,498)
Tupelo Young growth 0.33 1,014 (2,236)
Black 0.46 1,390 (3,064) Spruce
Water 0.46 1,385 (3,053) Engelmann 0.32 990 (2,184)
Walnut, black 0.51 1,536 (3,387) Red 0.38 1,112 (2,453)
Willow, black 0.34 1,086 (2,395) Sitka 0.37 1,112 (2,451)
Yellow-poplar 0.40 1,204 (2,655)
a
Based on weight when ovendried and green volume.
25. 21
Chapter 3
Air-Drying Process
The air drying of lumber involves exposing piles of stickered
lumber to the outdoor air. The dry-bulb temperature of the
air, its relative humidity as indicated by the wet-bulb depres-
sion, and the rate of air circulation are important factors in
successful air drying. Lumber in the pile dries most rapidly
when the temperature is high, the wet-bulb depression is
large, and air movement is brisk through the stickered layers
of lumber. The drying rate and the minimum moisture con-
tent attainable at any time in any place depend almost en-
tirely on the weather. Therefore, air drying of lumber cannot
be a closely controlled drying process.
Utilizing Air Movement
Green lumber dries because air conducts heat to the wood
surface and carries away the evaporated moisture. Thus, the
air must move within and through the lumber in the pile. As
warm, dry air enters the lumber pile, it takes up moisture
from the wood and the air temperature decreases. As the
cool, damp air leaves the pile, fresh air enters and drying
continues. The way air moves within a lumber pile depends
on the construction of the pile, its location within the yard,
and the yard layout and arrangement.
Boards are usually placed as close to edge-to-edge as possi-
ble in each course of a package, and there is limited opportu-
nity for the air to drop downward through the package as it
cools and becomes heavier. Consequently, the air must move
laterally across the boards. A downward movement of air can
be encouraged in lumber piles by building vertical flues,
chimneys, or spacing boards within the pile (Fig. 21). Air
enters the pile near the top or from the sides, passes over the
broad faces of the boards, cools, and drops downward
through the pile openings. A high and open pile foundation
permits the moist air to pass out readily from beneath the pile
and promotes the general downward movement of air. Chim-
neys are not necessary in kiln drying and are not commonly
used when packages go directly from the air-drying yard to
the kiln without restacking.
Boards in the upper parts of a pile dry quicker than do those
in the lower parts because the top boards are more exposed
to wind than are the lower boards. The air near the surface of
the ground and the bottom of the pile is generally cooler and
consequently has a higher relative humidity than the air near
the top of the pile. This cool, moist air entering the sides of
the lumber pile in the lower portions will not induce as rapid
drying as will the hotter, drier air entering the upper parts of
the lumber pile.
Factors That Influence
Drying Rate
The rate at which green lumber will dry after it is placed in
the air-drying yard depends on factors that involve the wood
itself, the pile, the yard, and climatic conditions.
Species
Some woods dry quickly, others slowly. Softwoods and some
lightweight hardwoods dry rapidly under favorable air-drying
conditions. The heavier hardwoods require longer drying
periods to reach the desired average moisture content. Spe-
cific gravity is a physical property of wood that can guide
estimations of drying rates or overall drying time. Southern
Pine will dry much faster in the yard than will southern oak.
In contrast, the lower density hardwoods like willow, yellow
poplar, and some of the gums will dry quickly. Sugar maple
will usually dry faster in northern yards than will northern red
oak. However, both species have about the same specific
gravity.
Thickness
One common rule of thumb is that drying time increases at a
rate of approximately the thickness raised to the 1.5 power.
This means that 50-mm- (2-in.-) thick lumber will require
about three times longer to lose a given amount of moisture
than will 25-mm- (1-in.-) thick lumber.
Grain Patterns
Quartersawn lumber dries more slowly than does flatsawn
lumber. Wood rays aid the movement of moisture, and in
quartersawn lumber, few wood rays are exposed on the broad
surfaces of the boards.
Sapwood and Heartwood
In softwoods, the moisture content of sapwood is usually
much greater than that of the heartwood (Table 1). However,
sapwood air dries faster. Usually it will be just as dry as the
heartwood, and perhaps drier, when the heartwood reaches
26. 22
the desired moisture content. In hardwoods, the sapwood
moisture content is often not much greater than the heart-
wood and is generally lower when air drying is terminated.
Piling Methods
The drying rate of lumber is affected by the way the boards
are stacked. For instance, lumber dries faster in air drying
when air spaces are left between the boards of a package than
when the boards are placed edge-to-edge. However, this is
not true in forced air circulation such as a dry kiln. Chimneys
built into packages promote better air flow through the lum-
ber stack, thus speeding up drying.
The drying yard can be opened up for faster drying by spac-
ing the piles farther apart. Placing piles on the outer fringes
of the yard will generally increase drying rate compared with
piles placed in the central portion of the yard.
Height and Type of Pile Foundation
Although downward air circulation within a pile is created by
natural convection, it is stimulated by winds carrying away
the cool moisture laden air underneath the pile. The founda-
tion under the pile should be reasonably high (460–610 mm;
18–24 in.) to allow the prevailing winds to create a brisk air
movement under the piles. Pile foundations should not ob-
struct air flow, and weeds or other debris should not be al-
lowed to block the air passage.
Yard Surface
The drying efficiency of a yard depends to some extent on
how well the ground surface is graded, paved, and drained
(Fig. 22). If water stands in a yard after a rain, it will de-
crease the drying rate and increase the risk of developing
stain. The yard should also be kept clean. Vegetation and
debris in the form of broken stickers, boards, or pieces of
timber from pile foundations interfere with the movement of
air over the ground surface. Vegetation, in particular, may
prevent air from passing out from beneath the bottom of the
piles and interfere with the circulation of air through the
lower courses of lumber.
Climate
The climate of the region in which the air-drying yard is
located can greatly influence the air-drying rate (Fig. 23).
Perhaps the most influential factor is temperature, but rela-
tive humidity and rainfall are also important factors. In
northern lumber-producing regions, the low temperature
retards the drying rate during the winter months. In the
southern part of the country, where the winter dry-bulb tem-
perature is higher, better drying conditions are expected.
Unfortunately, these higher temperatures may be offset,
Warm
air
in
Cool air out
Warm
air
in
Flue
(b)
Figure 21—(a) Random width boards in this hand-built
pile are spaced on each layer in addition to the chimney
to aid circulation within the pile; (b) shows air flow
through lumber pile and chimney.
Figure 22—Well-graded and surfaced main alleys aid
drainage and restrict growth of weeds.
27. 23
for instance in the Southeast, by rains that wet the lumber and
extend the drying time. In the Southwest, arid conditions can
make it difficult to keep degrade losses within bounds.
Typically, the drying rate does not change a great deal from
week to week, but in some areas hot, dry winds can acceler-
ate drying. Periods of high relative humidity may retard
drying. Precipitation is particularly important in air drying
lumber. Rewetting of the drying lumber results in a signifi-
cant decrease in the drying rate, and the added moisture must
be evaporated.
In addition to the general climatic conditions for a region,
local geographical features influence climatic conditions,
thereby influencing air-drying results. Yard sites at low
elevations, such as those near swamps or marshes or those
bordering on bodies of water, are likely to be damper than
surrounding areas. Such sites cause a decrease in drying rate
and encourage the development of mold, stain, and decay.
Elevated sites are more likely to be dry. Open sites, in con-
trast to those in valleys and those surrounded by tall trees,
hills, or buildings, are conducive to rapid drying because of
the greater movement of winds through them. Note that a
high, dry site encourages rapid drying but may cause surface
checking and end splitting in the lumber. Most importantly,
yearly variation of the weather can create a significantly
different drying rate compared with the expected average
drying rate.
Sunshine is also a factor. Solar radiation heats the land areas,
exposed areas of the lumber piles, and surrounding buildings.
Air moving over these warmed areas and structures is heated
and its drying potential increases. Black material absorbs
more solar energy and becomes hotter than light-colored
material. Thus, black topping the roadways and sometimes
the entire area can be advantageous in air-drying yards.
Drying Time and Final
Moisture Content
The time required to air dry lumber to a predetermined aver-
age moisture content depends not only on yard site, yard
layout, piling methods, and climatic conditions, but on spe-
cies, grain angle, and thickness. As previously mentioned,
lumber of low specific gravity will dry faster than will the
heavier woods. The approximate air-drying times for 25-mm
(1-in.) softwood and hardwood lumber (Table 7) are based
on climatic conditions for the region in which the particular
species is cut. These data are based on experience with hand-
stacked piles that vary in width from 2 to 5 m (6 to 16 ft).
Piles of lumber in packages, less than 2 m (6 ft wide), would
presumably dry in shorter periods. The minimum period
given applies to lumber piled during good drying weather,
generally during spring and summer. Lumber piled too late in
the period of good drying weather to reach 20% moisture
content, or lumber that is piled during the fall or winter,
usually will not reach a moisture content of 20% until the
following spring. This accounts for the maximum periods
given in Table 7. For example, 25-mm (1-in.) northern red
oak is often air dried to an average moisture content of about
Figure 23—Eastern (a) and western (b) United States showing the length of good air-drying conditions each year.
(Note: These maps are from different sources and do not agree exactly where they overlap.
28. 24
20% in southern Wisconsin in about 60 days when yarded in
June or July. If piled in the late fall and winter, the stock
must stay on sticks for 120 days or more. Local yard and
weather conditions and yard layout should be considered as
well as the seasonal weather pattern in estimating the time
required for air drying. Table 8 gives the air-drying times of
hardwood lumber in four regions of the United States. Drying
time estimates are available for a few species in a few loca-
tions (Table 9), where the drying time is estimated for lumber
stacked in each month of the year.
Table 7—Approximate time to air dry green 25-mm (1-in.) lumber to 20% moisture content
Species Time (days) Species Time (days)
Hardwoods Softwoods
Alder, red 20 to 180 Baldcypress 100 to 300
Ash Cedar
a
Black 60 to 200 Douglas-fir
Green 60 to 200 Coast 20 to 200
White 60 to 200 Interior north 20 to 180
Aspen Interior south 10 to 100
Bigtooth 50 to 150 Interior west 20 to 120
Quaking 50 to 150 Fir
a
Basswood, American 40 to 150 Hemlock
Beech, American 70 to 200 Eastern 90 to 200
Birch Western 60 to 200
Paper 40 to 200 Larch, western 60 to 120
Sweet 70 to 200 Pine
Yellow 70 to 200 Eastern white 60 to 200
Butternut 60 to 200 Jack 40 to 200
Cherry, Black 70 to 200 Lodgepole 15 to 150
Cottonwood Ponderosa 15 to 150
Black 60 to 150 Red 40 to 200
Eastern 50 to 150 Southern
Elm Loblolly 30 to 150
American 50 to 150 Longleaf 30 to 150
Rock 80 to 180 Shortleaf 30 to 150
Hackberry 30 to 150 Slash 30 to 150
Hickory 60 to 200 Sugar
Magnolia, Southern 40 to 150 Light 15 to 90
Maple Sinker 45 to 200
Bigleaf 60 to 180 Western white 15 to 150
Red 30 to 120 Redwood
Silver 30 to 120 Light 60 to 185
Sugar 50 to 200 Sinker 200 to 365
Oak Spruce
Northern red 70 to 200 Engelmann 20 to 120
Northern white 80 to 250 Red 30 to 120
Southern red 100 to 300 Sitka 40 to 150
Southern white (chestnut) 120 to 320 White 30 to 120
Pecan 60 to 200
Sweetgum
Heartwood 70 to 300
Sapwood 60 to 200
Sycamore, American 30 to 150
Tanoak 180 to 365
Tupelo
Black 70 to 200
Water 70 to 200
Walnut, black 70 to 200
Willow, black 30 to 150
Yellow-poplar 40 to 150
a
These species are usually kiln dried.
29. 25
Deterioration of Lumber
Losses in lumber value resulting from degrade that develops
during air drying are reflected in the overall cost of drying.
Air-drying degrade may be caused by shrinkage, fungal
infection, insect infestation, or chemical action. Shrinkage
causes surface checking, end checking and splitting, honey-
comb, and warp. Fungus infection causes blue or sap stain,
mold, and decay. Insect infestation results in damage as a
result of pith flecks, pinholes, and grub holes left in the
wood. Chemical reactions cause brown stain, gray stain, and
sticker marking. Ultraviolet radiation degrades the surface of
lumber and causes it to turn gray. However, this discoloration
is limited to a thin layer and can be planed off. If kept in the
yard for an extended time, the lumber may appear exces-
sively weathered because of an accumulation of sawdust and
windborne dirt.
Table 8—Estimated time to air dry green 25- and 50-mm (1- and 2-in.) eastern hardwood lumber to approximately
20% average moisture content
Estimated time by region (days)
Species
Size
(mm) South Mid-South Central Mid-North
Ash 25 45 to 70 45 to 75 45 to 80 60a
to 165
50 180 to 210 180 to 220 180 to 230 No data
Aspen 25 — — — 50a
to 120
50 — — — No data
Basswood, American 25 40 to 65 40 to 70 40 to 75 40a
to 120
50 170 to 200 170 to 210 170 to 220 No data
Beech, American 25 45 to 70 45 to 75 45 to 80 60a
to 165
50 180 to 210 180 to 220 180 to 230 No data
Birch, paper 25 — — — 40a
to 120
50 — — — 170 to 220
Birch, sweet; yellow 25 — 50 to 85 50 to 90 70a
to 165
50 — 190 to 240 190 to 250 No data
Butternut 25 — 40 to 70 40 to 75 60a
to 165
50 — 170 to 220 170 to 220 No data
Cherry 25 45 to 70 45 to 75 45 to 80 60a
to 165
50 180 to 210 180 to 220 180 to 230 No data
Cottonwood, eastern 25 40 to 65 40 to 70 40 to 75 50a
to 120
50 170 to 200 170 to 210 170 to 220 No data
Elm, American; slippery 25 40 to 65 40 to 70 40 to 75 50a
to 120
50 170 to 200 170 to 210 170 to 220 No data
Elm, rock; cedar; winged 25 50 to 80 50 to 85 50 to 90 80a
to 150
50 190 to 230 190 to 240 190 to 250 No data
Hackberry, sugarberry 25 40 to 65 40 to 70 40 to 75 30a
to 120
50 170 to 200 170 to 210 170 to 220 No data
Hickory 25 50 to 80 50 to 95 50 to 90 60a
to 165
50 190 to 230 190 to 240 190 to 250 No data
Magnolia 25 40 to 75 — — —
50 170 to 220 — — —
Maple, red; silver 25 40 to 65 40 to 70 40 to 75 30a
to 120
50 170 to 200 170 to 210 170 to 220 No data
Maple, sugar; black 25 45 to 70 45 to 75 45 to 80 50a
to 165
50 180 to 210 180 to 220 180 to 230 No data
Oak, lowland 25 100a
to 280 — — —
50 No data — — —
Oak, red (upland) 25 60 to 120 55 to 100 50 to 90 60a
to 165
50 240 to 360 215 to 300 190 to 250 No data
Oak, white (upland) 25 60 to 120 55 to 100 50 to 90 70a
to 200
50 240 to 360 215 to 300 190 to 250 No data
Pecan 25 60 to 120 65 to 100 50 to 90 60a
to 165
50 240 to 360 215 to 300 190 to 250 No data
Sweetgum, heartwood (red gum) 25 50 to 80 50 to 95 50 to 90 70a
to 200
50 190 to 230 180 to 240 190 to 250 No data
Sweetgum, sapwood (sap gum) 25 40 to 65 40 to 70 40 to 75 60a
to 165
50 170 to 200 170 to 210 170 to 220 No data
Sycamore 25 40 to 65 40 to 70 40 to 75 30a
to 120
50 170 to 200 170 to 210 170 to 220 No data
Tupelo (and blackgum) 25 60 to 110 45 to 90 45 to 80 70a
to 165
50 210 to 300 180 to 220 180 to 230 No data
Walnut, black 25 45 to 70 45 to 75 45 to 80 70a
to 165
50 180 to 210 180 to 220 180 to 230 No data
Willow, black 25 30 to 65 35 to 70 40 to 75 30a
to 120
50 150 to 200 160 to 210 170 to 220 No data
Yellow-poplar 25 40 to 65 40 to 70 40 to 75 40a
to 120
50 170 to 200 170 to 210 170 to 220 No data
a
To an average moisture content of 25%.
30. Table 9—Approximate days of air drying for lumber stacked in various locations at various times of the year to specified final moisture content
Air drying time (days)
Stack date
Northern
red oak
25 mm
(1 in)
Madison,
WI
20% MC
Northern
red oak
25 mm
(1 in)
Roanoke,
VA
a
Hard
maple
25 mm
(1 in)
Amasa,
MI
20% MC
Beech
25 mm
(1 in)
Philadel-
phia, PA
20% MC
Yellow
poplar
25 mm
(1 in)
Roanoke,
VA
20% MC
Ponderosa
pine
25 mm
(1 in)
Flagstaff,
AZ
15% MC
Ponderosa
pine
38 mm
(1 in)
Snowflake,
AZ
15% MC
Ponderosa
pine
38 mm
(1-1/2 in.)
Flagstaff,
AZ
15% MC
Ponderosa
pine
50 mm
(2 in.)
Snowflake,
AZ
15% MC
Sitka
spruce
25 mm
(1 in)
Vancouver,
BC
20% MC
Douglas-fir
25 mm
(1 in)
Everett,
WA
20% MC
Douglas-fir
50 mm
(2 in.)
Everett,
WA
20% MC
Western
hemlock
25 mm
(1 in)
Vancouver,
BC
20% MC
January 115 90 158 133
February 107 68 133 96
March 91 35 102 67
April 75 60 (21) 50 35 18 10 10 43 19 31 33 90 32
May 65 60 (21) 39 35 18 10 6 25 10 18 31 79 30
June 70 60 (20) 40 35 15 7 7 25 10 20 27 67 31
July 72 60 (21) 42 35 14 41 32 52 55 28 22 54 38
August 86 70 (25) 45 35 15 34 26 40 34 35 18 43 51
September 212 60 (28) 205 35 20 9 9 130 17 40 206 227 61
October 188 60 (30) 170 125 43 94 83 110 104 167 179 199 197
November 173 160 (22) 155 105 141 79 59 97 87 151 151 167 171
December 151 130 (22) 126 85 111 62 44 95 75 146 125 162 146
a
Final moisture content in parentheses.
31. 27
Chapter 4
Air-Drying Yard
The air-drying yard is generally located near the sawmill that
produces the lumber or close to the factory that uses the
wood for manufacturing finished products, and the yard is
usually laid out for convenient transport. The main roadways
or alleys of the air-drying yard are wide enough for the type
of lumber handling equipment being used (Fig. 24). Most
yarding operations build-up piles of lumber for air drying
with prestacked packages handled by a forklift truck or other
mechanized equipment. In some yards, piles have been built
by hand, either from the ground level or elevated trams or
docks.
Yard Layout and Orientation
An efficient yard layout should provide good drainage of rain
and melting snow, free movement of air in and out of the
yard, and easy transportation and piling of lumber. A yard
laid out for rapid drying potential should be on high, well-
drained ground with no obstructions to prevailing winds.
However, the need to keep the yard close to the plant limits
site selection, and convenient areas are not always favorable
to rapidly air drying lumber and providing a minimum of
degrade. Yard sites bounded by buildings or with standing
water or streams nearby should be avoided because this
retards lumber drying.
Most yards are laid out in a rectangular scheme. The alleys or
roadways cross each other at right angles, and the areas
occupied by the piles are rectangular. Specific areas may be
designated for certain species, grades, or thicknesses of
lumber. The alleys serve as routes for transporting the lum-
ber, as pathways for the movement of air through the yard,
and as protection against the spread of fire. The alleys in an
air-drying yard are classified as main and cross alleys
(Figs. 25, 26). Main alleys are for access to the lumber stacks
and cross alleys are for access to the main alleys. In large air-
drying yards, blocks or areas are often separated by still
wider roadways or strips of land to protect the lumber from
the spread of fire and to meet insurance requirements. These
wide alleys are sometimes called fire alleys.
The sides of the lumber piles are parallel to the main alleys,
which means air flows through the lumber stacks from one
main alley to an adjacent main alley. Spaces between the
sides and ends of the piles are also part of the yard layout.
These spaces form additional passageways for air movement.
Yards can be oriented in either of two ways. The main alleys
run north and south to obtain faster roadway drying after rain
or faster snow melt. With this orientation, the roadways are
exposed to solar radiation more than when the main alleys
are oriented east and west, where the lumber piles shade the
main alley roadways and retard roadway drying and snow
melt. This orientation might be best suited for areas of high
precipitation, especially snowfall. It is also desirable to orient
the main alleys as close as possible to parallel with the pre-
vailing winds. With this orientation, the wind can blow un-
impeded through the main alleys. As a result of complex
patterns of air velocities through adjacent main alleys, air
pressure varies within, which causes air to flow through the
lumber packages from one main alley to the next. If the
prevailing winds are directly west–east, it is impossible to
satisfy both the advantages of solar radiation and prevailing
winds. However, there is often a north–south component to
prevailing winds that does allow some air flow parallel to the
main alleys. The problem with north–south main alley orien-
tation and direct west–east winds is that drying from the first
row of lumber stacks upwind increases the relative humidity
of the air exiting that row, thus slowing the drying in rows
Figure 24—In an air-drying yard, the alleys must be wide
enough to enable the forklift to travel down the roadway
and maneuver the package onto the pile.
32. 28
further downwind. This orientation also results in upwind
rows blocking air flow to packages downwind.
Alley Size
Lumber piles are arranged either in rows of approximately
six piles (Fig. 25) or in lines of two piles (Fig. 26). The line
arrangement is used when the maximum air-drying rate is
desired because air only has to flow through two piles rather
than six as in the row arrangement. The main alleys are
generally 7 to 9 m (24 to 30 ft) wide. Cross alleys intersect
the main alleys at right angles and provide access to the main
alleys. Cross alleys also afford protection against the spread
of fire and may be 18 m (60 ft) or more in width and spaced
every 61 or 91 m (200 or 300 ft). In addition to providing
ample room for the forklift truck to maneuver in and out of
the rows, alleys must be wide enough to allow
clearance for the longest lumber being handled.
Row Spacing
Spaces between the ends of lumber piles aligned in rows
should be large enough so that a lift truck can operate easily.
The rows should be a minimum of about 1 m (3 ft) apart
(Fig. 25). In a line-type yard, the spacing between lines is
usually 0.6 m (2 ft).
Lateral Pile Spacing
The spacing between piles varies with differences in climate,
yard site, and the character of the lumber. Pile spacing also
varies with the different specific drying defects to be
avoided. Where surface checking is the defect most likely to
occur, the width of the spaces should be reduced. Where
staining is likely to occur, it is desirable to increase the spac-
ing. In arid regions, during the hot, dry season, piles should
be placed closer together than they are during the cool, moist
season. In a row-type yard, the space between the piles
within the rows should be about 0.6 m (2 ft) but may be as
7.2 m
(24 ft)
Cross alley
Cross alley
Mainalley
18m
(60ft)
Lateral space0.6 m
(2 ft)
Spacebetweenrows0.9m
(3ft)
15 m
(50 ft)
45m
(150ft)
Figure 25—General arrangement of a row-type
air-drying yard.
7.2 m
(24 ft)
Cross alley
Cross alley
Mainalley
Lateral space0.6 m
(2 ft)
Littlespacebetweenpiles
18m
(60ft)
Figure 26—General arrangement of
a line-type air-drying yard.
33. 29
much as 2.4 m (8 ft) in the middle of the row when variable
spacing is practiced. The spaces between the ends of piles in
a line-type yard are usually 0.3 to 0.6 m (1 to 2 ft).
If the lift truck has a side shifter, the space can be minimized
if air movement needs to be reduced.
Pile Width
For air drying, the width of the lumber pile varies from about
1 to 2.4 m (3-1/2 to 8 ft). Piles only 1 m (3-1/2 ft) wide
usually consist of several packages of lumber that are to be
placed on kiln trucks after air drying.
Pile Height
Pile height usually ranges from about 1.2 to 4.6 m
(4 to 15 ft). Sometimes 9-m- (30-ft-) high piles are built
using forklift trucks by stacking packages separated by
bolsters. High piles of narrow packages are often tied to-
gether with long bolsters for stability.
Yard Transportation Methods
In a forklift yard, the unit packages are already stickered
when they are carried out to the yard for piling. Bumping,
jarring, and rough handling will displace stickers. The most
practical remedial measure is to improve the roadways so
that transport can be rapid without jolting the stickered pack-
age. Some companies have found it advantageous to blacktop
the entire yard surface. Blacktop absorbs and stores solar
radiation and acts as a moisture vapor barrier against ground
water.
In yards where only the main and cross alleys are improved
roadways, the initial arrangement of the rows of piles within
blocks becomes permanent. The surface in the rows may be
graded and graveled.
Pile Foundations
A pile foundation, or pile bottom, supports the lumber pile
and provides clearance (Fig. 27) between the lumber and the
ground. However, pile foundations must allow the air that
has moved downward through the pile to be readily ex-
hausted. Fixed pile foundations are an integral part of the
yard layout, because they determine the location of the piles
or rows or lines of piles. Pile foundations represent a consid-
erable capital investment, and they should be well designed
and made of materials that will contribute to a long life and
low maintenance.
A pile foundation can consist of the following parts: mud
sills or sleepers, posts or piers, stringers, and crossbeams
(Fig. 28). Mud sills or sleepers rest on the surface of the
ground or slightly below the surface and support the piers or
posts. Wood mud sills should be pressure treated with a
preservative or be of the heartwood of a decay-resistant
species. When the entire surface of the yard is paved with
concrete or blacktop, mud sills are unnecessary, and the posts
or piers may rest directly on the pavement.
Posts or piers may be made from round or square pieces of
wood, concrete, cement building blocks, or masonry. Square
wood posts should be about 150 by 150 mm (6 by 6 in.) in
cross section, and round wood posts should be 150 to
200 mm (6 to 8 in.) in diameter. Diagonal bracing may be
fastened between the posts to prevent lateral tipping. When
posts or piers are set in the ground, they should extend
beneath the frost line and be supported by footings designed
to carry the estimated load. This arrangement requires
no bracing.
Cross beams may consist of wood timbers, steel angle irons,
or old rails and must be aligned with the stickers. If they are
made of wood, they should be 100 by 150 mm (4 by 6 in.) in
dimension and placed on edge. Stringers and cross beams are
not as susceptible to attack by decay fungi as are mud sills
and posts. However, stringers can decay where they come in
contact with the tops of the posts or with the cross beams. A
thorough brush treatment with a wood preservative at these
areas will reduce the decay hazard.
Pile foundations should not obstruct or block air movement
in any direction. An arrangement that restricts air movement
over the surface of the yard and from beneath the lumber
pile, such as laying several long planks or timbers on top of
one another to form foundations, is to be avoided. In this
instance, air beneath the pile is permitted to move only in the
direction parallel to the foundation timbers.
Forklift Yard
Pile foundations should be high enough to allow air drift
under the bottom package. Foundations for piles in a forklift
yard are usually level and often are movable or temporary.
These foundations frequently consist of short timbers, 150 by
150 mm (6 by 6 in.) in dimension, placed directly upon the
ground immediately before building the pile (Fig. 28).
Figure 27—The main features of well-constructed pile
foundations and lumber piles in an air-drying yard.
34. 30
Sometimes 150- by 230-mm (6- by 9-in.) preservative-treated
timbers are placed with the broad face on the ground. All
timbers except the outer ones are moved aside when the
forklift truck travels up and down the space for the row
of piles.
Lumber Pile Protection
One disadvantage of yard air drying is exposing lumber to
the weather. Exposure to direct sunshine and rain or melted
snow causes alternate drying and absorption of moisture,
which in turn causes checking and splitting. Roofs and covers
of various types can protect lumber while it dries and reduce
degrade and loss in value that will occur when exposed
alternately to liquid water and sunshine.
Pile Roofs or Covers
An effective pile roof is an essential feature of a good air-
drying practice. A roof protects the upper courses of lumber
and, to a lesser extent, the lower parts of the pile from direct
sunshine and precipitation. Without a roof, the lumber in the
upper courses, and particularly the top course, will warp,
check, and split. Rain or snow penetrating the pile may retard
drying, contribute to the development of fungus stains and
chemical sticker stain, and cause surface checks to increase
in size.
To afford maximum protection, a roof should project beyond
the ends and sides of the pile. For a level pile of packages,
the roof should project about 0.3 m (1 ft) at both ends. The
roof on a package does not usually project on the sides,
because the roof would be in the way of the forklift. A sloped
roof will permit the water to drain (Fig. 28).
Piles of lumber may be roofed with waterproof paper, build-
ing paper, or roll roofing laid directly on the top course of
lumber and weighted. These materials may also be combined
with boards to form a panel roof. A pile roof may consist of a
single panel or a pair. A single panel is usually designed to
slope from one end of the pile to the other. When two panels
are used, they should overlap at the center. Paper or roofing
felt provides water tightness, while the boards support the
paper or roofing in a flat sheet and permit the panel roof to
be anchored to the pile by clamps (Fig. 29) or other forms of
tie-downs. The narrowness of package piles encourages the
use of numerous materials in the design of pile roofs. Materi-
als such as exterior grade plywood, or hardboard, may be
used. Sheets of galvanized corrugated iron and corrugated
aluminum are also satisfactory; several roofing materials are
available. The choice of a suitable roofing material should be
based on the cost and expected life of the material. Pile roofs
can be reused many times, and their cost is small when
Figure 28—Features of a level pile of packages and the design of all wood movable foundations.
35. 31
compared with the cost of lumber degrade that would occur
if no pile covers were used.
Shade Cloth
Refractory woods, such as the oaks and beech, exposed to
hot, dry winds may surface check badly. By baffling the air
movement through the stickered lumber with shade cloth, the
surface EMC conditions are modified enough to reduce
surface checks and improve lumber brightness (Fig. 30). A
shade cloth also prevents rain from blowing into the pile.
End Coatings
Moisture-resistant coatings should be applied to the end
grain surfaces of green lumber to retard end drying and
minimize the formation of end checks and end splits. The
wood beneath the coating is maintained at a higher moisture
content than if the coating were not used. To be effective, the
coatings must be applied to the freshly trimmed green lumber
before any checking has started.
Many kinds of end coatings are commercially available, but
there are two basic types: those that are applied cold and
those that are applied hot. Cold coatings are most widely
used for lumber products. They are applied by swab, brush,
or spray and are often wax emulsions. It is important to
obtain a thick coating. If the end coating material is thinned
for easier application, additional coatings may be necessary.
The costs of an end coating and its application are often
justified by the value of the lumber saved.
Shed Drying
Lumber, particularly the more valuable grades, is sometimes
air dried in open sheds (Fig. 31). Drying sheds are usually
pole-type structures, although other structural materials such
as steel posts and metal trusses and roofing may be used. The
air-drying shed has a permanent roof so that the lumber is not
rewetted by rain. In areas or regions where rain wetting
unduly extends the drying time in a conventional air-drying
yard, shed drying reduces the time required to attain the
desired moisture content and maintain quality.
Sheds are generally open so that the sides and ends do not
obstruct air movement. Under certain conditions, shade cloth
may be used to form walls. It keeps the lumber clean and
reduces surface checking of refractory woods.
Shed-dried lumber is usually brighter in appearance than is
lumber air dried in a yard because weathering from ultravio-
let radiation or discoloration caused by rewetting is pre-
vented (Fig. 32). The shed roof usually extends beyond the
piles and protects the lumber from sunshine; therefore, end
checking and splitting are also greatly reduced.
Drying sheds are usually loaded by forklift trucks. Pile foun-
dation requirements are the same as for the forklift yard. The
shed floor may or may not be paved. If not, grading may be
required, depending upon soil conditions, and perhaps gravel
or crushed stone application can be justified.
The drying shed can be fairly wide to make up long rows of
piles within the bays. Entry to the rows would usually be
from both ends of the rows or both sides of the shed. In
contrast, the drying shed may be long and narrow with two
lines similar to a line-type forklift yard.
Figure 29—Loose sheets of corrugated metal are tied to
the package with a “C” clamp. A sticker is laid across
the top of the package on which the clamp is placed.
Figure 30—Shade cloth or any loose weave fabric can
be used in air drying and shed drying to reduce
degrade resulting from uncontrolled rapid drying
and exposure to the sun and rewetting.
